The crucible or mold used in arc melting furnaces is generally fabricated of copper or other material of similar high thermal conductivity for utilization of the electric-arc techniques to obtain the high melting temperatures required in forming ingots of refractory metals and alloys.
In the usual crucible or mold construction, an inner wall of copper defines the mold wall and, with an outer shell spaced from the copper inner wall, defines a cooling jacket through which flows a cooling fluid such as water or a glycerol solution to cool the mold during the normal course of furnace operation.
The necessary high temperature for melting refractory metals and alloys is provided by an electric arc struck between an anode and the surface of the sample to be melted in the crucible. An electric circuit is completed to form the arc by a positive charge of electricity delivered through the copper barrel of the crucible.
Reference is made throughout the specification amd claims to the use of copper in a crucible. As used in this context, the word "copper" includes all other metals and alloys used to make crucibles for electric arc furnaces.
The term "sample", as used herein, means the confirmation of the content of elements in a specified quantity of metals and alloys.
The arcing between the anode and the refractory sample in the copper crucible is necessary to melt the sample, but the high electrical conductivity of the prior art all-copper crucible rushes the protons toward the neutrons in the anode along the most direct, but indeterminate, path of least resistance. The result is frequent uncontrolled wayward arcing that breaks away fragments of the copper mold wall which undesirably become a part of the sample ingot. The copper fragments from the cavity wall mix with the sample ingot and prevent an accurate analysis because the sample ingot contains more copper than the mass of metal supposedly represented by the sample.
The scarring of the mold wall by wayward arcing is sometimes so frequent and severe as to shorten the useful life of the crucible. It has been known to replace crucibles two or three times a week at a cost of about $350.00 per crucible.
U.S. Pat. No. 3,078,529 issued Feb. 26, 1963 to Cooper, et al for MELTING CRUCIBLE AND COOLING MEANS THEREFORE illustrates one effort to solve the problem of damage from wayward arcing in an arc melting furnace. Cooper, et al uses a liquid metal cooling medium in place of the usual water employed in the cooling jacket of crucibles for arc melting. Use of water in the cooling jacket of crucibles of the cold mold type had been found objectionable because wayward arcing in the crucible occasionally breaks the mold wall and allows water from the cooling jacket to enter the crucible. Contact of water with a molten refractory sample can result in disastrous explosion. The explosion hazard is eliminated when water is replaced by liquid metal as the cooling medium, according to the invention of Cooper, et al.
U.S. Pat. No. 2,761,002 issued Aug. 28, 1956 to Laird, et al for SAFETY MOLD CHAMBER FOR ARC MELTING FURNACES provides an intermediate layer of gas maintained under pressure between the copper crucible and the cooling jacket. As an additional safety feature Laird, et al provides an arc-extinguishing relay which is activated when a wayward arc would damage the copper wall of the crucible.
These prior art attempts to solve the problem of crucible damage by wayward arcing accept wayward arcing as an inherent risk in the operation of electric arc furnaces and seek to minimize the effect by immunizing the mold wall or controlling the extent of the damage rather than seeking to control the cause of wayward arcing and thereby reduce the risk of mold damage.
The conventional copper mold is largely straight-walled with an inward taper near its lower end terminating in a flat bottom wall extending perpendicular to the side wall of the mold. This configuration has the disadvantage of sometimes trapping the sample after it cools, making it difficult to remove the sample from the mold.